H. Shinagawa

A two-dimensional MHD model of the solar wind interaction with Mars

The ionosphere of Mars is expected to be significantly affected by the solar wind because Mars does not possess a significant intrinsic magnetic field which deflects the solar wind. Despite a number of plasma measurements made near Mars, the nature of the solar wind-Mars interaction has not yet been fully understood. In order to self-consistently study the solar wind interaction with the ionosphere of Mars, a two-dimensional MHD model has been developed with an emphasis placed on the structure of the ionosphere of Mars. It is found that the modeled electron density profile in the upper ionosphere strongly depends on the solar wind dynamic pressure as well as the solar zenith angle. The ionosphere in the model tends to have an ionopause-like sharp drop of the electron density at some altitude for realistic solar wind dynamic pressures. Such behavior is not consistent with most of the observed electron density profiles, which exhibit relatively large and constant scale height in most of the dayside region. While the observed electron density profiles of the Venus ionosphere have been reproduced reasonably well by ionospheric models as well as recent three-dimensional MHD models, the electron density profiles of the Martian ionosphere have not been successfully reproduced by theoretical models including this study. This fact implies that processes not present in the Venus ionosphere, such as crustal magnetic fields and the rotation of the planet, may have significant effects on the structure and the dynamics of the ionosphere of Mars.

Dynamics of Neutral Wind in the polar region observed with two Fabry-Perot Interferometers

Optical observations were made at Ramfjord, Norway from January 10 to February 14,1997. Two types of Fabry-Perot interferometers (FPIs), Doppler-imaging and scanning, were installed at the EISCAT radar site and were used to acquire data simultaneously with radio instruments. Both FPIs can observe emissions of two different wavelengths simultaneously. We can estimate the horizontal and vertical wind in different emission layers simultaneously with high time-resolution (∼1 min). The observations on February 8 and 9, 1997, show some notable characteristics: (1) large-scale perturbations (≈ ±150 m/s) are observed in the upper thermospheric wind. They seem to begin 30 min after the onset of a magnetic substorm and to stop when the next substorm begins. (2) Clear wave-like structures are found in the horizontal wind variations. Some of them can be seen over the entire sky, and one of them is found in a restricted regions. (3) A clear wave-like structure is also found in the vertical wind in the upper thermosphere. A similar structure can be seen in the lower thermosphere, but these structures are not always in phase. This phases difference starts at the same time that horizontal winds between the two layer has their phase difference. (4) The relation between the vertical wind and the divergence of horizontal wind seems to change with time. The correlation coefficient between them changes one-hours before and on-time of a substorm on-set. This sign of the coefficient is negative in most of the time, with considering about time-lag. It means the vertical morion is caused by divergent flow of horizontal wind.

Our current understanding of the ionosphere of Mars

Abstract Despite a number of observations near Mars, the structure and dynamics of the ionosphere of Mars have not been fully understood mainly because of an insufficient amount of magnetic field data within the ionosphere. In 1997, Mars Global Surveyor (MGS) successfully observed magnetic fields within the ionosphere for the first time. MGS discovered fairly strong ( B ≤ 1600 nT) but localized magnetic fields in various regions on the Martian surface. Those magnetic fields are interpreted as crustal magnetic anomalies. At the same time, strength of a global intrinsic magnetic field of core origin was found to be smaller than about 5 nT at the surface of Mars. A Venus-like ionospheric magnetic field induced by the solar wind was also seen in the ionosphere of Mars. The results suggest that the Martian ionosphere is controlled both by solar wind interaction and by the crustal magnetic field. Therefore, the nature of the Martian ionosphere is probably different from any other planetary ionospheres, and is likely to be most complicated among the planetary ionospheres. Current understanding of the ionospheres of Mars is reviewed, and outstanding problems with the Martian ionosphere are pointed out.

Thermospheric and ionospheric dynamics in the auroral region

Abstract Behavior of the thermosphere and the ionosphere in the auroral region, especially near an auroral arc, is quite complicated. There have been a number of reports on vertical neutral winds in the auroral region, suggesting that heat sources associated with local auroral activities could cause extremely large upwelling and downwelling of the thermosphere. A two-dimensional nonhydrostatic thermosphere-ionosphere model is used to study variations in the thermosphere and the ionosphere associated with a moving auroral arc. The results are compared with data obtained by the EISCAT (European Incoherent Scatter) radar measured in the vicinity of auroral arcs. The overall behavior of the observed ion motion parallel to magnetic field lines is interpreted as perturbation caused by neutral wind driven by auroral arcs. The model calculation suggests that temporal and spatial variations in the heating region significantly influence the structure of neutral and ion motion near auroral arcs.

The Mars thermosphere-ionosphere: Predictions for the arrival of Planet-B

The primary science objective of the Planet-B mission to Mars is to study the Martian upper atmosphere-ionosphere system and its interaction with the solar wind. An improved knowledge of the Martian magnetic field (whether it is induced or intrinsic) is needed, and will be provided by Planet-B. In addition, a proper characterization of the neutral thermosphere structure is essential to place the various plasma observations in context. The Neutral Mass Spectrometer (NMS) onboard Planet-B will provide the required neutral density information over the altitude range of 150–500 km. Much can be learned in advance of Planet-B data taking as multi-dimensional thermosphere-ionosphere and MHD models are exercised to predict the Mars near-space environment that might be expected during the solar maximum conditions of Cycle 23 (1999–2001). Global model simulations of the Mars thermosphere-ionosphere system are presented and analyzed in this paper. These Mars predictions pertain to the time of Planet-B arrival in October 1999 (F10.7∼200; Ls∼220). In particular, the National Center for Atmospheric Research (NCAR) Mars Thermosphere General Circulation Model (MTGCM) is exercised to calculate thermospheric neutral densities (CO2, CO, N2, O, Ar, O2), photochemical ions (CO+2, O+2, O+ below 200 km), neutral temperatures, and 3-components winds over 70–300 km. Cases are run with and without dust loading of the lower atmosphere in order to examine the potential impacts of dust storms on the thermosphere-ionosphere structure. Significant dust-driven impacts are predicted in the lower thermosphere (100–120 km), but are less pronounced above 150 km. The ionospheric peak height changes greatly with the passage of a Mars global dust storm event. In addition, Martian dayside exobase temperatures are generally warmer during dusty periods, in accord with Mariner 9 UVS data (Stewart et al., 1972). During the Planet-B mission, the NMS team intends to use the MTGCM as a facility tool whose simulated output can be utilized to aid various investigations.

Field-aligned ion motions in the polar E-F transition region: Mean characteristics

[1] Geomagnetic field-aligned (FA) ion motions in the transition region between E and F regions (from 150 to 250 km; hereafter termed the E-F transition region) in the polar ionosphere are analyzed statistically using data from European Incoherent Scatter (EISCAT) radar from 1987 to 1999. We use all available EISCAT data sets that satisfy the definition of data selection criteria (i.e., 24-hour observation periods beginning at 1200 UT); therefore it has not been feasible to investigate seasonal, solar, and geomagnetical activity dependences. FA ion motions above and below the E-F transition region (250-300 km and 100-150 km, respectively) are also analyzed to compare with lower-altitude ion motions in the E-F transition region. The FA ion velocity that is observed with the EISCAT radar can be written as the sum of the FA ion diffusion velocity and FA component of the thermospheric wind using the ion momentum equation. The main purpose of this paper is to understand quantitatively the relative contributions of ion diffusion and neutral wind on FA ion motions in the E-F transition region. We derive the amplitude and phase of the first three daily harmonics, i.e., the 24-, 12-, and 8-hour periodic oscillations in observed FA ion velocities and estimated FA ion diffusion velocities. The spectral characteristics are determined with a Fast Fourier Transfer (FFT) method after averaging velocity data in I -hour bins. The amplitude of the 24-hour periodic oscillation is the largest of the three harmonic components for the FA ion velocity as well as FA ion diffusion velocity. While the amplitude of the 24-hour periodic oscillation in FA ion diffusion velocity below 230 km is considerably smaller than the values of the observed FA ion velocities, the amplitude from 230 to 300 km is comparable to that in observed FA ion velocities. From 230 to 300 km the phase of the 24-hour periodic oscillation in FA ion diffusion velocity has about 180° difference from that in the observed FA ion velocity. This spectral analysis results suggest that, in the E-F transition region, FA ion diffusion velocity is not the major contributor to the three harmonic oscillations in the observed FA ion velocities; thus FA component of thermospheric winds or tides prove to be the primary contributor.

The geospace environment data analysis system

Abstract This paper presents our recent efforts installing a high-technology computer system, called GEDAS, at the Solar-Terrestrial Environment Laboratory, allowing us to conduct integrated studies combining ground-based and satellite-based observations and simulation research. GEDAS lets researchers/engineers around the world communicate instantaneously and will actively be involved in researching and predicting space weather. We demonstrate in this paper some of the on-going core projects, including predictions of major geomagnetic storms based on solar wind information, as well as specifications of the magnetosphere-ionosphere coupling system. It is planned that data products generated from real-time original data and simulation models can be nearly instantly provided to the world community via GEDAS.

Effects of auroral arcs on the generation of gravity waves in the auroral F-region

Abstract The relationship between auroral activity and the generation of gravity waves (GWs) in the auroral F -region (150–300 km) was investigated using data from the European Incoherent Scatter (EISCAT) radar and an all-sky camera at Kilpisjarvi (69.0° N, 20.8° E). The oscillations of neutrals observed with the EISCAT radar on 1 March 1995 had a dominant oscillation period of 24 min, which is longer than the typical Brunt-Vaisala period in the auroral F -region (≅ 13 min). According to the equation of the dispersion relation for GWs, the horizontal phase-velocity and the horizontal wavelength of the observed oscillations were about 110 m/s and about 160 km, respectively. The observed oscillations showed a downward propagation of the phase with time. Although we could not find conclusive evidence that the observed oscillations were GWs, the estimated characteristics of the observed oscillations were typical ones for the medium-scale GWs. The all-sky images showed that the auroral arc extended in an almost zonal direction near the source region estimated from the values of the observed wave-parameters. When the oscillations were compared with ones calculated using the Francis model, the phase lines of the modeled GWs showed agreement with those of the observed oscillations. Although we could not rule out the possibility of generating the observed oscillations by meteorological sources, enhancements of electromagnetic energy in association with the arc are considered to be strong candidate source to generate the observed oscillations.

Thermospheric temperature and density variations

The thermosphere is the transition region from the atmosphere to space. Both the solar ultraviolet radiation and the solar wind energy inputs have caused significant thermospheric variations from past to present. In order to understand thermospheric/ionospheric disturbances in association with changes in solar activity, observational and modelling efforts have been made by many researchers. Recent satellite observations, e.g., the satellite CHAMP, have revealed mass density variations in the upper thermosphere. The thermospheric temperature, wind, and composition variations have been also investigated with general/global circulation models (GCMs) which include forcings due to the solar wind energy inputs and the lower atmospheric effects. In particular, we have developed a GCM which covers all the atmospheric regions, troposphere, stratosphere, mesosphere, and thermosphere, to describe variations of the thermospheric temperature and density caused by both effects from the lower atmosphere and the magnetosphere. GCM simulations represent global and localized temperature and density structures, which vary from hour to hour, depending on forcings due to the lower atmosphere, solar and geomagnetic activities. This modelling attempt will enable us to describe the thermospheric weather influenced by solar activity in cooperation with ground-based and satellite observations.

MHD and Kinetic Modeling of the Ionospheres of Venus and Mars

It is widely recognized that both Venus and Mars possess no significant global intrinsic magnetic fields, and that the solar wind interacts directly with the upper atmospheres and ionospheres of Venus and Mars. In addition, local crustal magnetic fields are also present in various regions at Mars, suggesting that some regions of the Martian ionosphere are influenced not only by the solar wind but also by the crustal magnetic field. Previous studies have suggested that the basic structures of the ionospheres of the planets can be described by fluid and MHD(magnetohydrodynamic) processes. Various models of the ionospheres of Venus and Mars based on the MHD formulation have been constructed during the last two decades. Although the MHD approach has been successful in reproducing the ionospheres of the planets, some studies have indicated that MHD modeling is not necessarily appropriate in the regions of the topside ionosphere, the ionopause, and the magnetosheath, where the ion kinetic processes are likely t...